Cholesteric liquid crystal display system

ABSTRACT

A display system includes a display arranged to receive an image wise pattern of light to form an image, including, a pair of conductors, at least one conductor being transparent, a layer of cholesteric liquid crystal material disposed between the conductors, the liquid crystal material having multiple stable optical states at zero electrical field, and a light absorber for forming an image wise thermal pattern in the liquid crystal sufficient to change the optical state of the cholesteric liquid crystal in response to an image wise pattern of light; a display writer, including, a light source for producing a flash of light of sufficient intensity to generate sufficient heat in the light absorber to change the optical state of the cholesteric liquid crystal, a mask located between the light source and the display for defining the image wise pattern of light, a display drive connectable to the conductors for generating an electric field between the conductors for changing the optical state of the cholesteric liquid crystal, and a controller connected to the light source and the display drive for controlling the intensity of the electrical field and actuating the light source to create an image on the display.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a divisional application of U.S. patent application Ser. No.10/256,930 filed Sep. 27, 2002, now U.S. Pat. No. 6,885,409corresponding to U.S. Application Publication No. US 2004/0061808A1.

FIELD OF THE INVENTION

The present invention relates to a display system having a polymerdispersed cholesteric liquid crystal that change optical states inresponse to heat, light and electrical field.

BACKGROUND OF THE INVENTION

Cholesteric liquid crystals have the property of maintaining severaldifferent optical states in the absence of electrical field.Additionally, cholesteric liquid crystals can change optical states inresponse to applied electrical and/or thermal fields. Those propertiesmake them useful in the development of field-stable, re-writabledisplays.

U.S. Pat. No. 3,401,262 issued Sep. 10, 1968 to Fergason et al.discloses a cathode ray tube to apply light to a screen. The screen hasa photoconductive layer that is excited by an electrical field appliedby fine leads across the photoconductive layer. The screen has a layerof a temperature sensitive cholesteric material that changes reflectivewavelength with slight changes in temperature, and changes hue in heatedareas. Light from the cathode ray tube strikes the photoconductor layer,creating heat which can be used to selectively change the color of thesheet of cholesteric material. The system uses a complex cathode raytube and a photoconductor layer and ceases to present an image in theabsence of an electrical field.

U.S. Pat. No. 3,578,844 issued May 18, 1971 to Churchill discloses asheet of gelatin encapsulated cholesteric material without aphotosensitive layer. The sheet is put into a first reflective state byheating. Portions of the sheet are written into a black (clear) state bythe application of DC fields. The sheet is heated to reset the display.The encapsulated material in the sheet retained written informationwithout fade at ambient conditions for eight weeks.

U.S. Pat. No. 3,789,225 issued Jan. 29, 1974 to Leder discloses a glassycholesteric liquid crystal between glass plates. Glassy liquid crystalmaterials are solidified liquid crystals in an orderly state at ambienttemperatures. They are not responsive to electrical fields in the glassystate. The apparatus writes the sheet to an initial state by heating thematerial above the isotropic (liquid) transition point. As the materialis cooled, a high-intensity xenon flash lamp is used to disturb thematerial so that flash disturbed areas solidify into a state differentthan areas not receiving flash energy. The imaging system requires thatthe materials be raised to a high temperature, and cooled at a fast ratein the presence of selective high-intensity flash light. No electricalfields are applied to the media.

Conventional, non-glassy liquid crystals have the property of beingelectrically driven between a planar state reflecting a specific visiblewavelength of light and a light scattering focal-conic state at ambienttemperatures. Chiral nematic liquid crystals, also known as cholestericliquid crystals have the capacity of maintaining one of multiple givenstates in the absence of an electric field. U.S. Pat. No. 5,437,811issued Aug. 1, 1995 to Doane et al. discloses a light-modulating cellhaving a polymer dispersed chiral nematic liquid crystal. The chiralnematic liquid crystal has the property of being driven between a planarstate reflecting a specific visible wavelength of light and a weaklylight scattering focal-conic state. Chiral nematic liquid crystals, alsoknown as cholesteric liquid crystals, have the capacity of maintainingone of multiple given states in the absence of an electric field. TheDoane et al. patent discloses the use of only electrical fields tochange the optical state of cholesteric liquid crystals. The technologywrites image data line sequentially. Sequentially writing data lines isslow compared to writing all pixels at once and requires electricaldrivers on each column and row line.

Yamamoto et al. in A Novel Photoaddressable Electronic Paper UtilizingCholesteric LC Microcapsules and Organic Photoconductor, SID 2001DIGEST, pp. 362-365, create an electronic paper having a photoconductivelayer and a polymer encapsulated cholesteric liquid crystal that isfield responsive at ambient temperatures. A high electrical field isapplied across both layers, and the photoconductive layer provides abias voltage in the presence of light. The high and low field statesacross the material write cholesteric material into different opticalstates.

Prior art light sensitive sheets have required expensive and complexphotosensitive layers for operation. Electrical drive systems must writedata sequentially, requiring complex electronic drives. Glassy liquidcrystals change state with the application of large amounts of heat andno electrical field. There is a need therefore for a light written sheetthat could have image data written simultaneously without aphotosensitive layer at low temperatures.

SUMMARY OF THE INVENTION

The need is met according to the present invention by providing adisplay system that includes a display arranged to receive an image wisepattern of light to form an image, including a pair of conductors, atleast one conductor being transparent; a layer of cholesteric liquidcrystal material disposed between the conductors, the cholesteric liquidcrystal material having multiple stable optical states at zeroelectrical field; and a light absorber for forming an image wise thermalpattern in the liquid crystal sufficient to change the optical state ofthe cholesteric liquid crystal in response to an image wise pattern oflight; a display writer, including, a light source for producing a flashof light of sufficient intensity to generate sufficient heat in thelight absorber to change the optical state of the liquid crystal, a masklocated between the light source and the display for defining the imagewise pattern of light, a display drive connectable to the conductors forgenerating an electric field between the conductors for changing theoptical state of the liquid crystal, and a controller connected to thelight source and the display drive for controlling the intensity of theelectrical field and actuating the light source to create an image onthe display.

ADVANTAGES

The present invention has the advantage that it provides a simple sheetstructure that can be written and re-written using a xenon flash lampand an electrical field. A single writing process can be used to writethe sheet without regard to prior written information. The writingprocess is fast, and improves image quality over sheets written onlyelectrically.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a partial cross sectional view of a display in accordance withthe present invention;

FIG. 2 is a schematic side view of a prior art chiral nematic materialin a planar and focal-conic state responding to incident light;

FIG. 3 is schematic side view of an experimental setup used to simulatea display system in accordance with the present invention;

FIG. 4 is a plot of the response of the display of FIG. 1, originally inthe planar state, at constant flash lamp energy and various voltages;

FIG. 5 is a plot of the contrast ratio of the data of FIG. 4;

FIG. 6 is a plot of the response of the display of FIG. 1, originally inthe focal-conic state, at constant flash lamp energy and variousvoltages;

FIG. 7 is a plot of the contrast ratio of the data of FIG. 6;

FIG. 8 is an electrical schematic diagram for a display writer inaccordance with the present invention;

FIG. 9 is a side view of the display of FIG. 1 attached to an object;

FIG. 10 is a side schematic view of a display writer in accordance withthe present invention;

FIG. 11 is a side view of the display writer connected to a display towrite the display in accordance with the present invention; and

FIGS. 12A and B are front views of a display according to the presentinvention in prewritten state and written states, respectively.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring to FIG. 1, a display 10 according to the present inventionincludes a display substrate 15, such as a thin transparent polymericmaterial, for example, Kodak Estar film base formed of polyester plasticthat has a thickness of between 20 and 200 (preferably 125 microns).Other polymers, such as transparent polycarbonate, can also be used.

A first transparent conductor 20 is formed on display substrate 15.First transparent conductor 20 can be tin-oxide, indium-tin-oxide (ITO),or polythiophene, with ITO being the preferred material. Typically thematerial of first transparent conductor 20 is sputtered or coated as alayer over display substrate 15 having a resistance of less than 1000ohms per square.

In a preferred embodiment, a first conductor cover 22 is printed overfirst transparent conductor 20. First conductor cover 22 can be screenprinted conductive ink such as Electrodag 423SS screen printableelectrical conductive material from Acheson Corporation. Such screenprintable conductive materials comprise finely divided graphiteparticles in a thermoplastic resin. First conductor cover 22 protectsfirst transparent conductor 20 from abrasion.

Light modulating layer 30 overlays a first portion of first transparentconductor 20. A portion of light modulating layer 30 is removed tocreate exposed first conductor 20′ to permit electrical contact. Lightmodulating layer 30 contains cholesteric liquid crystal material, suchas those disclosed in U.S. Pat. No. 5,695,682 issued Dec. 9, 1997 toDoane et al., the disclosure of which is incorporated by reference.Application of electrical fields of various intensity and duration canbe employed to drive a chiral nematic material (cholesteric) into areflective state, to a substantially transparent state, or anintermediate state. These materials have the advantage of having firstand second optical states that are both stable in the absence of anelectrical field. The materials can maintain a given optical stateindefinitely after the field is removed. Cholesteric liquid crystalmaterials can be Merck BL112, BL118 or BL126, available from E.M.Industries of Hawthorne, N.Y.

In a preferred embodiment, light modulating layer 30 is E.M. Industries'cholesteric material BL-118 dispersed in deionized photographic gelatin.The liquid crystal material is mixed at 8% concentration in a 5% gelatinaqueous solution. The liquid crystal material is dispersed to create anemulsion having 8-10 micron diameter domains of the liquid crystal inaqueous suspension. The domains can be formed using the limitedcoalescence technique described in U.S. Pat. No. 6,423,368 issued Jul.23, 2002 to Stephenson et al. The emulsion is coated on a polyesterdisplay substrate over the first transparent conductor(s) and dried toprovide an approximately 9-micron thick polymer dispersed cholestericcoating. Other organic binders such as polyvinyl alcohol (PVA) orpolyethylene oxide (PEO) can be used in place of the gelatin. Suchemulsions are machine coatable using coating equipment of the typeemployed in the manufacture of photographic films. A gel sub layer canbe applied over the first transparent conductor 20 prior to applyinglight modulating layer 30 as disclosed copending U.S. Ser. No.09/915,441 filed Jul. 26, 2001 by Stephenson et al.

FIG. 2 is a schematic side sectional view of a chiral nematic materialin a planar and focal-conic state responding to incident light. In thefigure on the left, after a high voltage field has been applied andquickly switched to zero potential, the liquid crystal molecules becomeplanar liquid crystal 72, which reflect portions of incident light 60 asreflected light 62. In the figure on the right side of FIG. 2, uponapplication of a lower voltage field, the molecules of the chiralnematic material break into weakly forward scattering cells known asfocal-conic liquid crystal 74. Increasing the time duration of alow-voltage pulse progressively drives the molecules that wereoriginally reflective planar liquid crystal 72 towards a fully evolvedand light scattering focal-conic liquid crystal 74.

A light absorber 35 is positioned on the side opposing the incidentlight 60. Light absorber 35 can be a thin layer of light absorbing,sub-micron carbon in a gel binder as disclosed copending U.S. Ser. No.10/036,149 filed Dec. 26, 2001 by Stephenson. As fully evolvedfocal-conic liquid crystal 74, the cholesteric liquid crystal is forwardlight scattering and incident light 60 is absorbed by light absorber 35to create a black image. Progressive evolution towards the focal-conicstate causes a viewer to perceive reflected light 62 that transitions toblack as the cholesteric material changes from reflective planar liquidcrystal 72 to a fully evolved light scattering focal-conic liquidcrystal 74. When the field is removed, light modulating layer 30maintains a given optical state indefinitely. The states are more fullydiscussed in U.S. Pat. No. 5,437,811, referenced above.

Returning to FIG. 1, light absorber 35 is disposed between secondconductor 40 and light modulating layer 30 to improve contrast. A secondconductor 40 overlays light modulating layer 30. Second conductor 40 hassufficient conductivity to provide an electric field between the firsttransparent conductor 20 and second conductor 40 strong enough to changethe optical state of the cholesteric material in light modulating layer30. Second conductor 40 can be formed, for example, by the well knowntechnique of vacuum deposition for forming a layer of conductivematerial such as aluminum, tin, silver, platinum, carbon, tungsten,molybdenum, tin or indium or combinations thereof. The layer ofconductive material can be patterned using well known techniques ofphotolithography, laser etching or by application through a mask.

In a preferred embodiment, second conductor 40 is formed by screenprinting a conductive ink such as Electrodag 423SS screen printableelectrical conductive material from Acheson Corporation. Such screenprintable conductive materials comprise finely divided graphiteparticles in a thermoplastic resin. Screen printing is preferred tominimize the cost of manufacturing the display.

The use of a flexible support for display substrate 15; firsttransparent conductor 20; machine coated light absorber 35 and lightmodulating layer 30; and printed second conductor 40 and first conductorcover 22 permits the fabrication of a low cost flexible display. Smalldisplays according to the present invention can be used aselectronically rewritable tags for inexpensive, limited rewriteapplications.

Referring to FIG. 3, a display 10 in accordance with the preferredembodiment was positioned so that a conventional xenon flash 52 exposedportions of display 10 through mask 54. In the experiment, flash 52 wasa Vivitar model 285HV professional flash lamp and mask 54 was a sheet ofDupont Mylar transparency with an electro-photographic printed image.The output of flash 52 could be adjusted to imprint an image on display10 if the cholesteric material was initially either planar liquidcrystal 72 or focal-conic liquid crystal 74.

A set of electrodes was applied to first conductor cover 22 and secondconductor 40. An electrical field was applied across electrodes 50, andflash imprinted images on display 10 were erased. Display 10 could beimprinted and erased multiple times without damage to display 10.Display 10 was positioned so that the black second conductor 40 facedflash 52 and mask 54. Flash 52 could be adjusted so that images wereimprinted through second conductor 40. Images made in the reversedmanner could be electrically erased using a field across electrodes 50.From these experiments, it was concluded that the printing processoccurs due to thermal energy being applied to second conductor 40through either side of display 10. A heat pulse of correct power andduration provides a thermal flux effect that can write cholestericmaterial into either the planar or focal-conic state. These experimentsare the first demonstration of the use of masked high-intensity light tothermally print and reprint images on polymer dispersed cholestericliquid crystals. The method and materials permit multiple erasure andwriting cycles. Furthermore, the structure of display 10 is flexible andlow-cost.

Experiments were performed to determine the response of display 10 tothe application of electrical fields during the thermal heat pulses fromflash 52. In a first experiment, display 10 was electrically writteninto the planar state using a high voltage pulse. The output of flash 52was set so initially planar liquid crystal was selectively written intothe focal-conic state in unmasked areas. The display was repeatedlyreset to the planar state, and written using a series of voltages.

FIG. 4 shows a plot of the response of the display of FIG. 1, originallyin the planar state, at constant flash lamp energy and various voltages.For each test, the material was initialized into the planar using a highvoltage pulse. Then a test voltage was applied during a flash event. Theresulting curve is equivalent to the response curve found in the Doaneet al. patent for materials tested without flash. The curve for themasked material (filled boxes) is the same response of materials in theabsence of the thermal pulse provided by flash 52. The clear curve isthe same display in the presence of the heat pulse from the flashcombined with an electrical field. At zero applied voltage and without amask, planar material is written into the focal conic-state and has areflectance of about 7 percent. Applying a low voltage, such as 10volts, improves the clarity of focal-conic state from 7 percent to about2 percent reflectance.

FIG. 5 shows a plot of the contrast ratio of the data of FIG. 4. Thecombination of the flash and electrical field creates two high contraststates which are improved from a standard 4:1 contrast ratio to a 14:1contrast ratio. The two optimal states are optically reversed states.One optimum contrast ratio, 14:1, occurs at 20 volts. Material initiallyin the planar state is unaffected by the low-voltage pulse without thepresence of the flash energy; the presence of energy from flash 52writes the material into the focal-conic state. A second high-contraststate, 13:1 contrast ratio occurs at 60 volts applied field. Without theflash energy, initially planar material is written into the focal-conicstate; the presence of the flash energy writes the material into theplanar state.

FIG. 6 is a plot of the response of the display of FIG. 1, originally inthe focal-conic state, at constant flash lamp energy and variousvoltages. FIG. 7 is a plot of the contrast ratio of the data of FIG. 6.Application of a low field again reduces the reflection of focal-conicmaterial from 7 percent to 2 percent. Only one pair of bistable statesis possible in this system, again at 60 volts, which has a contrastratio of about 13. Material initially in the focal-conic state remainsin the focal-conic state at a medium level voltage. Energy from flash 52causes initially focal-conic material to be driven into the planarstate. Apparently, heat from flash 52 reduces the voltage required todrive cholesteric material into the planar state.

It was observed that at 60 volts of applied field, the final state ofthe material was defined by mask 54 if the material was initially ineither the planar or focal-conic state. The phenomenon eliminates theneed to initially write the material into an initial state beforeflash-writing an image. The single writing process, without aninitialization step, provides a fast, parallel method of writing display10.

The multiple states of the system suggest a variety of drive schemes,shown in Table 1. A first scheme (1) initializes display 10 into theplanar state using either a 100 volt pulse (1A) or a 60 volt pulse withflash (1B). A low, 20 volt, pulse in conjunction with the flash permitsselective writing using mask 54. In a second scheme (2), display 10 iswritten into the focal conic state either without flash (2A) or with aflash (2B). In scheme 2, 60 volts is applied to display 10, and areasreceiving flash light are written into the planar state, and areas thatare masked remain in the focal-conic state. In a third scheme (3) isessentially identical to scheme 2B, but eliminates initialization to afocal-conic state. A 60 volt pulse in the presence of flash energyprovides image-wise writing of display 10 using mask 54 withoutinitialization. The use of an initialization step may be useful inapplications that require display 10 to be erased in a first operation,and a later re-writing of display 10.

TABLE 1 Initial Initial Initial Write SCHEME Flash voltage state VoltageMasked Clear 1A No 100 Planar 20 Planar F-C 1B Yes 60 Planar 20 PlanarF-C 2A No 60 F-C 60 F-C Planar 2B Yes 20 F-C 60 F-C Planar 3 — — XXXX 60F-C Planar

FIG. 8 is an electrical schematic for a display writer made inaccordance with the present invention. A power supply 91 provides powerto a flash capacitor 92 and to the display drive 93. A masking display94 is disposed to selectively mask the output of flash 52. Maskingdisplay 94 can be a simple twisted-nematic (TN) or super-twisted-nematic(STN) display of conventional design. Controller 95 supplies informationto masking display 94. Controller 95 applies writing voltage toelectrodes 50 through display drive 93, connected to display 10. Atrigger circuit 96 triggers flash 52 in conjunction with the applicationof a bipolar electrical field from display drive 93. The flash energy ismasked by masking display 94 to apply an image-wise light pattern fromflash lamp 52 in conjunction with an applied field to write an image ondisplay 10.

In an experiment, a dot-matrix super-twisted-nematic (STN) display, partnumber TM 13164 BCHG-1 from Tianma Corporation in Taiwan was placed overa display 10 which was built in accordance to the preferred embodiment.A Vivitar flash, already described, was adjusted so that light absorbingportions of the STN display masked the flash and transparent portions ofthe STN display passed flash light. The masking effect was sufficient towrite areas of display 10 into the focal-conic or planar statesdepending on the optical state of the masking display 94. The flash unitwas discharged through the STN display repeatedly with no observableharm to the structure of the STN display or display 10. The experimentshows that it is possible to use simple, low-cost STN displays asmasking display 94.

FIG. 9 is a side view of the display of FIG. 1 attached to an object 80.Object 80 can be a tray that supports display 10 or an article to whichdisplay 10 is attached, such as a card. Display 10 is attached to object80 using contacts 82. Contacts 82 can be an electrically conductiveadhesive having two areas, a first area in contact with firsttransparent conductor 20 and a second area in contact with secondconductor 40. In FIG. 9, the left portion is connected to firsttransparent conductor 20 and the right portion is connected to secondconductor 40. Contacts 82 are connection areas for electrodes 50 toconnect to display 10 as well as an adhesive to secure display 10 toobject 80.

FIG. 10 is a side schematic view of a separable writer in accordancewith the present invention. In the preferred embodiment, writer 90 isconnected to display 10 for a writing event. Writer 90 contains theelectrical components of FIG. 8. Writer 90 has a housing (not shown)which positions flash 52, masking display 94 and electrodes 50 inpositions to write a display 10. Details (not shown) of conventionaldesign in object 80 and the housing of writer 90 align writer 90 todisplay 10.

FIG. 11 is a side section view of the writer connected to the display towrite display 10 in accordance with the present invention. Writer 90 isconnected to display 10 when electrodes 50 are pressed against contacts82. Sensors (not shown) of conventional design can be connected tocontroller 95 to signal that writer 90 is connected to display 10.Sensors can also be provided to signal controller 95 to write an imageto display 10. The writing method, previously disclosed, can be used towrite an image on display 10. Writer 90 can be detached from display 10,and used to write other displays 10. Object 80 has attached informationon display 10 that has been updated.

FIG. 12A shows a display 10 according to the present invention in aprewritten state. When display 10 is manufactured according to thepresent invention, the material is uniformly in the planar state.Alternatively, display 10 may have been written and can have areashaving cholesteric material in either a focal-conic or planar state, orany intermediate state. Alternatively, previous information may havebeen erased by writer 90 in a previous writing event. Returning to FIG.11, writer 90 is brought into a writable position with respect todisplay 10. In that position, electrodes 50 contact areas of contacts 82which are connected to first transparent conductor 20 and secondconductor 40. Using method 3, no initialization writing is performed. Amasking display 94 in writer 90 receives image data from controller 92.Controller 92 applies a field through electrodes 50 to display 10 anddischarges flash 52. Radiation from flash 52 selectively heats areas ofdisplay 10 in the presence of an electric field to write an image ondisplay 10. Areas of display 10 receiving light are written into theplanar state and areas masked from light from flash 52 are written intothe focal conic state regardless of initial state of the cholestericliquid crystal. FIG. 12B is a display according to the present inventionin a written state. Brighter, lighter areas have been written into theplanar state and the darker areas have been written into the focal-conicstate.

The method and apparatus of this invention provides low-cost, simple andrapidly written labels with high information content. The displays areinexpensive, having a few simple, un-patterned, mass produced layers.The writing apparatus itself is simple, requiring a low-cost flashsystem, an inexpensive masking display and a 2 wire electrical exciterfor the display 10. Separating the drive from the display permits manyinexpensive labels to be written by a single, simple display writer 90.

The invention has been described in detail with particular reference tocertain preferred embodiments thereof, but it will be understood thatvariations and modifications can be effected within the spirit and scopeof the invention.

PARTS LIST

-   10 display-   15 display substrate-   20 first transparent conductor-   20′ exposed first conductor-   22 first conductor cover-   30 light modulating layer-   35 light absorber-   40 second conductor-   50 electrodes-   52 flash-   54 mask-   60 incident light-   62 reflected light-   72 planar liquid crystal-   74 focal-conic liquid crystal-   80 object-   82 contacts-   90 writer-   91 power supply-   92 flash capacitor-   93 display drive-   94 masking display-   95 controller-   96 trigger circuit

1. A method of writing and erasing an image, comprising (a) providing alayer of polymer dispersed cholesteric liquid crystal material disposedbetween a pair of unpatterned conductors and a light absorber forforming an image wise thermal pattern in response to an image wisepattern of light; (b) applying an electrical field having a firstmagnitude across the unpatterned conductors to erase any image in thepolymer dispersed cholesteric liquid crystal material and initializingthe material; and (c) applying an electrical field having a secondmagnitude across the unpatterned conductors and exposing the layer ofpolymer dispersed cholesteric liquid crystal material to an image wiseflash of light to write the image.
 2. The method of claim 1, wherein acombination of the applying the electric field having the secondmagnitude and the exposing via the image wise flash of light provides acontrast ratio with respect to a state change caused by the combinationin the polymer dispersed cholesteric liquid crystal material that isgreater than the contrast ratio otherwise provided by a light sourcewithout a flash.
 3. The method of claim 1, wherein the exposing via theimage wise flash to write the image comprises causing a state change inthe polymer dispersed cholesteric liquid crystal material via the imagewise flash.
 4. A method of writing and erasing an image on a display,comprising: (a) providing a display comprising a pair of conductors, atleast one conductor being transparent, for forming an electrical fieldthere between; a layer of polymer dispersed cholesteric liquid crystalmaterial disposed between the conductors, the polymer dispersedcholesteric liquid crystal material having multiple stable opticalstates at zero electrical field and responsive to an electric fieldbetween the conductors for changing optical state; and a light absorberfor forming an image wise thermal pattern in the polymer dispersedcholesteric liquid crystal sufficient to change the optical state of thepolymer dispersed cholesteric liquid crystal to display the image; and(b) applying a first electrical field across the conductors and exposingthe layer of polymer dispersed cholesteric liquid crystal material to animage wise flash of light to write the image.
 5. The method of claim 4,further comprising applying a second electrical field across theconductors to erase any image in the polymer dispersed cholestericliquid crystal material and initialize the material before applying thefirst electrical field.
 6. The method of claim 5, wherein a flash oflight is applied with the second electrical field.
 7. The method ofclaim 5, wherein the second electrical field has the same or greatermagnitude than the first electrical field.
 8. The method of claim 4,wherein a combination of the applying the first electrical field and theexposing via the image wise flash of light provides a contrast ratiowith respect to a state change caused by the combination in the polymerdispersed cholesteric liquid crystal material that is greater than thecontrast ratio otherwise provided by a light source without a flash. 9.The method of claim 4, wherein the exposing via the image wise flash towrite the image comprises causing a state change in the polymerdispersed cholesteric liquid crystal material via the image wise flash.